先看代码，复制使用即可，以下代码均可复制粘贴使用（我将以注释的形式解释代码左右，您将看到以下效果）。
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<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title></title>
<style type="text/css">
.div1 {
width: 0px;
height: 0px;
border: 50px solid;
border-color: transparent;
border-top-color: red;
}
.div2 {
width: 0px;
height: 0px;
border: 50px solid;
border-color: transparent;
border-right-color: red;
}
.div3 {
width: 0px;
height: 0px;
border: 50px solid;
border-color: transparent;
border-bottom-color: red;
}
.div4 {
width: 0px;
height: 0px;
border: 50px solid;
border-color: transparent;
border-left-color: red;
}
</style>
</head>
<body>
<div class="div1"></div>
<div class="div2"></div>
<div class="div3"></div>
<div class="div4"></div>
</body>
</html>
附加和可能出现的有用信息已添加注释